1. Technical Field of the Invention
The present invention relates to circuit boards, and more particularly to a multilayer printed circuit board which includes conducting layers of lightweight material.
2. Description of the Prior Art
Recent progress in electronics technology has resulted in electronic circuits capable of performing increasingly sophisticated functions. A large number of these electronic circuits can now be implemented on a single chip to form a single compact integrated circuit. The use of integrated circuits has allowed the design and manufacture of sophisticated electronic products of, smaller size, and increasingly lighter weight. In order to design and manufacture these sophisticated electronic products, it is necessary that many integrated circuits be mounted and electrically interconnected in a single compact package using some type of high density packaging. Because the input/output pin count of integrated circuit devices has increased correspondingly with the sophistication of the functions contained on the chip, the high density packaging arrangement must also allow the electrical interconnection between a large number of pins from many integrated circuits. Multilayer printed circuit boards are typically used for this high density packaging.
Multilayer printed circuit boards are made up of a number of conducting layers or laminae of printed circuitry between which are placed insulating layers or laminae. The conducting layers are formed of a predetermined pattern, in which portions of the conducting material have been removed. Various methods are known in the art for fabricating multilayer circuit boards. In one common method, the predetermined patterns are formed by etching away conducting material from a conducting layer, after conducting material has been deposited on an insulating layer to form a layer of the board. The layers of the multilayer printed circuit board, each of which has been prepared in this way, are attached to each other by lamination, adhesive such as acrylic adhesive, or other appropriate means. A conducting layer is placed as the outer conducting layer on at least one surface--and typically both surfaces--of the board. The remaining conducting layers form the inner conducting layers of the board. Electrical components, including integrated circuits, are mounted on the side, or, sides of the multilayer printed circuit board which contain an outer conducting layer. The input/output pins of these integrated circuits and components are directly connected to the outer conducting layer of the surface on which they are mounted. Electrical connections between pins of the components mounted on the multilayer printed circuit board's surface are made through the predetermined patterns of the various conducting layers. When it is necessary to connect two different conducting layers to form a desired connection, the connections are done via holes drilled through the insulating and conducting layers. These holes are plated on their inner surface with metalization to form connection towers that provide a electrical connection between certain conducting layers.
A typical multilayer printed circuit board includes six conducting layers of Cu, approximately 0.025 mm thick, each separated from the other by an insulating layer of a material, such as a 0.10-0.29 mm thick layer of FR4. The metalization plating on the holes forming the connection tower that connects the conducting layers is also Cu. The Cu-Cu same material contacts provide non-corrosive, low resistance contacts within the multilayer printed circuit board. Also, the Cu conducting layer on the surface of the multilayer printed circuit board allows low resistance contacts to be made to devices external to the board.
When used in a low weight portable device, such as a portable computer or personal digital assistant(PDA), the surface area necessary for component mounting on a multilayer printed circuit board ranges, for example, from 967 cm.sup.2 to 1290 cm.sup.2. Since two sides of the board are normally used to mount components, a multilayer printed circuit board of from 483 cm.sup.2 to 645 cm.sup.2 normally required. The weight of a multilayer printed circuit board of this size could be large compared to the total weight of the portable device. Using 561 cm.sup.2 as a typical one-sided surface area of a double-sided circuit board, the weight of a representative multilayer printed circuit board can be calculated. The specific gravity of Cu is 8.900 g/cm.sup.3. The specific gravity of FR4 is 2.100 g/cm.sup.3. The six Cu layers weigh 6.times.(561 cm.sup.2 .times.0.0025 cmx (8.900 g/cm.sup.3)) for 74.894 g. The five FR4 layers weigh 5.times.(561 cm.sup.2 .times.0.010 cmx (2.100 g/cm.sup.3)) or 58.905 g. Ignoring the weight of the connection towers, the weight of the multilayer printed circuit board is 133.799 g. or approximately 4.7 ounces. In a portable electronic device such as a 28 ounce PDA, 4.7 ounces is a significant percentage of the total weight.
The use of lighter weight conducting materials in place of the Cu conducting layers of the multilayer printed circuit board can reduce the weight of the board. However, in order to provide noncorrosive low resistance contacts to the components on the surface of the board it is desirable to use Cu for the surface conducting layer. The use of Cu solely for the surface conducting layers, in a multilayer printed circuit board in which lighter weight conducting materials, such as Al are substituted for the inner conducting layers, creates a problem in that noncorrosive low resistance electrical contacts are difficult to make between the outer and inner conducting layers. For example, Cu has an electrochemical potential of 0.34 V, and Al has an electrochemical potential of -1.66 V. A contact between Cu and Al that is exposed to the atmosphere when moisture is present is subject to corrosion when electric current passes through the contact. The exposure of Al to oxygen creates resistive aluminum oxide. As the corrosion builds up the resistance of the contact to current flow also increases.
The present method of using plated holes or bores for interconnecting conducting layers only provides a direct exposed contact between the plating material and conducting layer material. A direct contact of Cu plating material to a lighter weight material such as Al, which is exposed to the atmosphere does not provide the type of low resistance noncorrosive contact necessary for multilayer printed circuit boards.
It would provide an advantage to have a multilayer printed circuit board comprising conducting layers of different weight material, with low resistance noncorrosive electrical contacts between the conducting layers. This would allow lighter weight material to be used to replace heavier weight material, in appropriate conducting layers, to form a low weight multilayer printed circuit board. This low weight multilayer circuit board could be used in lightweight portable devices.